Showing papers on "Silica fume published in 2010"

Comparison of test methods to assess pozzolanic activity

Abstract: Assessment of the pozzolanic activity of cement replacement materials is increasingly important because of the need for more sustainable cementitious products. The pozzolanic activity of metakaolin, silica fume, coal fly ash, incinerated sewage sludge ash and sand have been compared using the Frattini test, the saturated lime test and the strength activity index test. There was significant correlation between the strength activity index test and the Frattini test results, but the results from these tests did not correlate with the saturated lime test results. The mass ratio of Ca(OH)2 to test pozzolan is an important parameter. In the Frattini test and strength activity index test the ratio is approximately 1:1, whereas in the saturated lime test the ratio is 0.15:1. This explains why the saturated lime test shows higher removal of Ca(OH)2 and why the results from this test do not correlate with the other test methods.

Use of natural zeolite as a supplementary cementitious material

Abstract: Natural zeolite, a type of frame-structured hydrated aluminosilicate mineral, is used abundantly as a type of natural pozzolanic material in some regions of the world. In this work, the effectiveness of a locally quarried zeolite in enhancing mechanical and durability properties of concrete is evaluated and is also compared with other pozzolanic admixtures. The experimental tests included three parts: In the first part, the pozzolanic reactivity of natural zeolite and silica fume were examined by a thermogravimetric method. In this case, the results indicated that natural zeolite was not as reactive as silica fume but it showed a good pozzolanic reactivity. In the second part, zeolite and silica fume were substituted for cement in different proportions in concrete mixtures, and several physical and durability tests of concrete were performed. These experimental tests included slump, compressive strength, water absorption, oxygen permeability, chloride diffusion, and electrical resistivity of concrete. Based on these results, the performance of concretes containing different contents of zeolite improved and even were comparable to or better than that of concretes prepared with silica fume replacements in some cases. Finally, a comparative study on effect of zeolite and fly ash on limiting ASR expansion of mortar was performed according to ASTM C 1260 and ASTM C 1567. Expansion tests on mortar prisms showed that zeolite is as effective as fly ash to prevent deleterious expansion due to ASR.

Mechanical properties of reactive powder concrete containing high volumes of ground granulated blast furnace slag

Abstract: The mechanical properties (flexural strength, compressive strength, toughness and fracture energy) of steel microfiber reinforced reactive powder concrete (RPC) were investigated under different curing conditions (standard, autoclave and steam curing). Portland cement was replaced with ground granulated blast furnace slag (GGBFS) at 20%, 40% and 60%. Sintered bauxite, granite and quartz were used as aggregates in different series. The compressive strength of high volume GGBFS RPC was over 250 MPa after autoclaving. When an external pressure was applied during setting and hardening stages, compressive strength reached up to 400 MPa. The amount of silica fume can be decreased with increasing amount of GGBFS. SEM micrographs revealed the tobermorite after autoclave curing.

Silica fume as porogent agent in geo-materials at low temperature

Abstract: The synthesis of geopolymers based on alkaline polysialate was achieved at low temperature (∼25–80 °C) by the alkaline activation of raw minerals and silica fume. The materials were prepared from a solution containing dehydroxylated kaolinite and alkaline hydroxide pellets dissolved in potassium silicate. Then the mixture was transferred to a polyethylene mold sealed with a top and placed in an oven at 70 °C for 24 h. For all geopolymer materials, following dissolution of the raw materials, a polycondensation reaction was used to form the amorphous solid, which was studied by FTIR-ATR spectroscopy. The in situ inorganic foam based on silica fume was synthesized from the in situ gaseous production of dihydrogen due to oxidation of free silicon (content in the silica fume) by water in alkaline medium, which was confirmed via TGA-MS experiments. This foam has potential as an insulating material for applications in building materials since the thermal measurement has a value of 0.22 W m−1 K−1.

Contribution of Rice Husk Ash to the Properties of Mortar and Concrete: A Review

Abstract: In the last decade, the use of supplementary cementing materials has become an integral part of high strength and high performance concrete mix design. These can be natural materials, by-products or industrial wastes, or the ones requiring less energy and time to produce. Some of the commonly used supplementary cementing materials are fly ash, Silica Fume (SF), Ground Granulated Blast Furnace Slag (GGBFS) and Rice Husk Ash (RHA) etc. RHA is a by-product material obtained from the combustion of rice husk which consists of non-crystalline silicon dioxide with high specific surface area and high pozzolanic reactivity. It is used as pozzolanic material in mortar and concrete, and has demonstrated significant influence in improving the mechanical and durability properties of mortar and concrete. This paper presents an overview of the work carried out on the use of RHA as partial replacement of cement in mortar and concrete. Reported properties in this study are the mechanical, durability and fresh properties of mortar/concrete.

Behavior and modeling of confined high-strength concrete

Abstract: This paper presents a study on the behavior and modeling of the stress-strain behavior of confined high-strength concrete (HSC) without silica fume. The behavior of actively confined HSC is first examined, and a unified active-confinement model applicable to both HSC and normal-strength concrete (NSC) is then proposed based on a large test database assembled from the existing literature. An experimental study on fiber-reinforced polymer (FRP)-confined HSC is next presented and interpreted to examine its behavior, forming the basis for the subsequent modeling work. It is eventually shown that a recent analysis-oriented model developed by the writers’ group for NSC also provides close predictions for FRP-confined HSC. While the work is primarily concerned with HSC without silica fume, the effect of incorporating silica fume into HSC on the behavior of confined HSC is also given appropriate attention. The presence of silica fume in HSC is shown to reduce the effectiveness of confinement in term of strain cap...

Comparative Study of the Effects of Microsilica and Nanosilica in Concrete

Abstract: It is well recognized that the use of mineral admixtures such as silica fume enhances the strength and durability of concrete. This research compares the effects of adding silica fume and nanosilica to concrete and provides a better understanding of the changes in the concrete nanostructure. Nanoindentation with scanning probe microscopy imaging was used to measure the local mechanical properties of cement pastes with 0% and 15% replacement of cement with silica fume. A reduction in the volume fraction of calcium hydroxide in a sample with silica fume provides evidence of pozzolanic reaction. Furthermore, replacing 15% cement by silica fume increased the volume fraction of the high-stiffness calcium silicate hydrate (C-S-H) by a small percentage that was comparable with the decrease in the volume fraction of calcium hydroxide. A parallel study of cement pastes with nanosilica showed that nanosilica significantly improves durability of concrete. This research provides insight into the effects of nanosilica on cement paste nanostructure and explains its effect on durability of concrete. The nanoindentation study showed that the volume fraction of the high-stiffness C-S-H gel increased significantly with addition of nanosilica. Nanoindentation results of cement paste samples with similar percentages of silica fume and nanosilica were compared. Samples with nanosilica had almost twice the amount of high-stiffness C-S-H as the samples with silica fume.

Combined effect of chemical nature and fineness of mineral powders on Portland cement hydration

Abstract: This paper focuses on the influence of the chemical nature and the fineness of the fillers on the hydration process and on the compressive strength development. Four different types of fillers are considered in combination with Portland cement: quartzite filler, alumina filler, limestone filler, and silica fume. The study deals with blended mortars having a 0.45 water to powder (cement and filler) ratio with a 10% substitution of cement by filler. Quartzite fillers do not seem to accelerate the hydration process in a significant way. No positive effect is noticed on the strength development either. The presence of a fine inert alumina powder increases the rate of early hydration of Portland cement. The greater the fineness, the faster the rate of hydration heat development. This reactivity leads to an increase in the compressive strength at early age for mortar containing the finest alumina powders. In case of coarse alumina powder, no acceleration effect is obtained. Finely ground limestone (calcite) fillers promote heterogeneous nucleation of hydrates which significantly accelerates hydration. At early age, this also results in an increased mortar compressive strength in comparison with the control mortar. From the obtained results, it is clear that both chemical natures as well as fineness are important with regard to the accelerating effect of the hydration process. With increasing fineness, the accelerating effect increases. For powders with comparable fineness, it is clear that limestone powder has a more significant accelerating effect than silica fume and alumina filler. Quartzite filler seems to have no significant effect.

Compressive strength, microstructure and thermal analysis of autoclaved and air cured structural lightweight concrete made with coal bottom ash and silica fume

Abstract: This research investigated the compressive strength, microstructure and thermal analysis of autoclaved and air cured structural lightweight concrete made with coal bottom ash and silica fume. The results show that bottom ash lightweight concrete autoclaved for 6 h gives compressive strength similar to the bottom ash lightweight concrete air cured for 28 days and found that the compressive strength of both bottom ash lightweight concrete increased when silica fume was added to the mix. The highest compressive strength obtained for all mixes was found when coal bottom ash was used at 20% with the addition of silica fume at 5% and that this strength value is significantly higher than that of Portland cement control. The thermal conductivity of all bottom ash lightweight concrete at 28 days and those autoclaved for 6 h were found to be slightly higher than that of Portland cement control concrete. Air cued hydration products such as ettringite, calcium silicate hydrate and gehlenite hydrate were detected using thermogravimetric analysis. The tobermorite phase detected in autoclaved bottom ash concrete with silica fume was found to give denser microstructure than the fibrous-like C–S–H phases detected in Portland cement control concrete.

Applicability of the standard specifications of ASTM C618 for evaluation of natural pozzolans

Abstract: Nowadays, the production of binary cements, containing pozzolans (including silica fume, fly ash, natural pozzolans, etc.), is a global practice. Many countries have ample resources of natural pozzolans, capable of being used in binary cements, which reduce environmental impacts while reaping greater economies of scale. The ASTM C618 standard provides one of the most applicable methods for evaluating natural pozzolans. Some research results show contradictions between performance of pozzolans in concrete and the specifications of ASTM C618, as pozzolans having a high pozzolanic activity according to this method do not always exhibit suitable performance in concrete, however, in the other researches ASTM C618 showed compatibility. This treatment analyses the chemical and physical properties of different natural pozzolans by means of ASTM C618 and complementary tests, viz. methods of EN 196-5, X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) studies, insoluble residue content and thermo-gravimetric investigations. Measurements of mechanical and transport properties, for concretes containing various fractions of the pozzolans, were performed for further verification. The results illustrate that the pozzolanic properties, determined via the ASTM C618 standard, show some disparities with the performance of concretes, whereas the EN 196-5 standard agrees well with performance.

Thermal analysis and microstructure of Portland cement-fly ash-silica fume pastes

Abstract: Thermal analysis (thermogravimetry and differential thermal analysis) was used with scanning electron microscopy technique to investigate the hydration mechanisms and the microstructure of Portland cement-Fly ash-silica fume mixes. Calcium silicate hydrate (C–S–H), ettringite, gehlenite hydrate (C2ASH8), calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3) phases were detected in all mixes. In the mixes with the use of silica fume addition, there is a decrease in Ca(OH)2 with increasing silica fume content at 5 and 10% compared to that of the reference Portland-fly ash cement paste and a corresponding increase in calcium silicate hydrate (C–S–H).

Prediction of the Compressive Strength of High Performance Concrete Mix using Tree Based Modeling

Abstract: is the safest and sustainable construction material which is most widely used in the world as it provides superior fire resistance, gains strength over time and gives an extremely long service life. Its annual consumption is estimated between 21 and 31 billion tones. Designing a concrete mix involves the process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible. According to the National Council for Cement and Building Materials (NCBM), New Delhi, the compressive strength of concrete is governed generally, by the water-cement ratio. The mineral admixtures like fly ash, ground granulated blast furnace, silica fume and fine aggregates also influence it. The main purpose of this paper is to predict the compressive strength of the high performance concrete by using classification algorithms like Multilayer Perceptron, M5P Tree models and Linear Regression. The result from this study suggests that tree based models perform remarkably well in predicting the compressive strength of the concrete mix.

Role of water film thickness in rheology of CSF mortar

Abstract: In a recent study, the authors have demonstrated that the combined effects of water content, packing density and solid surface area on the rheology of cement–sand mortar may be evaluated in terms of the water film thickness (WFT). The present study aims to extend the concept of WFT to mortar containing condensed silica fume (CSF). For the study, mortar samples with various CSF and water contents were made for packing density, flowability and rheology measurements. It was found that although the effects of adding CSF are fairly complicated, the WFT is still the single most important parameter governing the rheology of CSF mortar. However, the rheological properties are dependent also on the CSF content. Correlations of the rheological properties to both the WFT and CSF content yielded R2 values of at least 0.896.